This invention relates to high speed printers and more particularly to such printers employing an endless type carrier coacting with a number of sets of hammers, one hammer for each character position in a line and a hammer driver for each set of hammers, the hammer driver being timed-shared among the hammers of a set.
Impact line printers are characterized by a moving endless print carrier having a number of characters etched, engraved, embossed or otherwise formed on or attached to the carrier surface. The character area of the carrier is designed for coacting with a print hammer at each character position on a line to effect printing on a print medium such as paper. Storage, control and timing circuitry are utilized to actuate a particular hammer at the precise time that a character desired to be printed at a character position is in registration with the position.
Both drum and endless chain printers are common in the prior art. Drum printers employ a drum with a matrix of print characters, constantly rotating about its horizontal axis, and having identical type characters in any particular row. Each column defines a complete type font.
Chain printers employ an endless band having affixed thereto individual type elements. The band moves horizontally across the print medium proximate to the line to be printed. Usually a number of type fonts are presented successively along the chain.
The human eye is sensitive to vertical misalignment but relatively insensitive to horizontal misregistration. Drum type printers are sensitive to vertical misalignment, expensive to manufacture, assemble and maintain.
In chain printers typically, links of the type elements readily loosen, and further the band will often stretch or buckle.
Hammer firing must be precisely timed in impact printers to prevent ghosting, smudging or misregistration of printed characters. Therefore, hammer drive circuits must deliver extremely accurate actuation pulses, precise in rise time, duration and magnitude. Further, such pulses be most accurately timed to actuate the hammer exactly when printing is required.
The type of hammer which generally has been used is an electromagnetically operated device which is normally in a rest position and upon the receipt of an actuation pulse is moved from rest to a position of impact. The hammer forces the paper and type ribbon against the type character. High power levels are necessary to impart energy to the hammer during a substantial portion of hammer flight time.
If the printer is to operate at speeds compatible with contemporary computers, a number of hammers must be fired simultaneously or in rapid succession. This requires power supplies having high energy storage and quick recovery time. Such supplies are extremely costly, large and heavy.
Numerous attempts have been made to reduce the cost of printers by time-sharing hammers and/or hammer drive circuits. However, these attempts have resulted in a decrease in speed of operation, an increase in the capacity of the power supply and an increase in the complexity and capacity of the control, timing and storage circuitry.
For example, if hammer drivers are time-shared, once the drivers have been gated to fire at certain character positions, and complete their firing cycle, other characters to be printed have already passed their character position and printing must await the next appearance of the character. If four hammers share a single driver, four cycles of type font may occur before completing a line of type.
In some prior art systems, the type font has been displaced slightly so that characters arrive at various positions in timed sequence. However, hammer actuation pulses must be delivered so rapidly and have such high energy levels, that power supplies must be very large to recharge in time for the next firing cycle, heat dissipation and burn out becomes a problem, and false actuation of hammers often occurs.
Other systems have included shifting type bars, shifting paper carriers and like mechanical systems. These arrangements are limited by the low speed of operation of the mechanical devices employed.